For a variety of workpieces (e.g., semiconductor wafers, optical blanks, memory disks, etc.), manufacture requires the substantial planarization of at least one major workpiece surface. For ease of description and understanding, the following description will concentrate on exemplary embodiments of the present invention pertinent to semiconductor wafers. It should be understood, however, that the inventive carrier head may be utilized to planarize a wide variety of workpieces in addition to semiconductor wafers. Furthermore, as appearing herein, the term “planarization” is used in its broadest sense and includes any chemical and/or mechanical process that may be utilized to smooth (e.g., remove irregular topographical features from, change the thickness of, etc.) or polish the surface of a workpiece.
The technique of chemical mechanical polishing, also known as chemical mechanical planarization (referred to herein collectively as “CMP”), has been widely adopted for the planarization of semiconductor wafers. CMP processes produce a substantially smooth, planar face along a major surface of the wafer (referred to herein as the wafer's front surface) to prepare the workpiece surface for subsequent fabrication processes (e.g., photoresist coating, pattern definition, etc.). During CMP, an unprocessed wafer is transferred to a carrier head, which then presses the wafer against a polishing surface (e.g., a polish pad) supported by a platen. Polishing slurry is introduced between the wafer's front surface and the polish pad (e.g., via conduits provided through the polish pad), and relative motion (e.g., rotational, orbital, and/or linear) is initiated between the polish pad and the wafer carrier. The mechanical abrasion of the polish pad and the chemical interaction of the slurry produce a substantially planar topography along the wafer's front surface.
One known type of carrier head generally includes a flexible membrane or bladder that contacts the back (i.e., the unpolished) surface of the work piece during the CMP process. The bladder may be secured to the carrier head by way of a plurality of clamp rings threadably coupled to bolts extending through the carrier head housing. Multiple pressure chambers or plenums are provided behind the bladder to form a number of annular pressure zones across the bladder's working face. The pressure within each zone is independently adjusted to vary the force applied to the wafer's back surface at different locations. The CMP apparatus may be provided with an induction system (e.g., a closed-loop eddy current system) to monitor the topographical features of the wafer's front surface during polishing/planarization. For example, the induction system may identify thicker wafer surface areas requiring a higher rate of removal, and the pressure within the zone or zones corresponding to the thicker surface areas may be increased accordingly. After a major surface of the wafer has been satisfactorily planarized, the carrier head ejects the wafer by, for example, expanding a central portion of the bladder to physically force the wafer away therefrom (commonly referred to as “bullfrogging”).
Despite extensive engineering, conventional carrier heads are still limited in certain respects. For example, the utilization of multiple clamp rings and bolts to attach the bladder to the carrier head housing increases the overall complexity and weight of the carrier head and further complicates the task of refurbishing the carrier head (e.g., replacing exhausted bladders). Moreover, the tightening of each bolt may produce a relatively high and localized clamping force. Consequently, large portions of the carrier head (e.g., the carrier head housing) must typically made of a metal capable of withstanding high axial forces without deformation. The manufacture of the carrier head housing and other carrier head components from metal not only increases the weight of the carrier head, but may also lead to carrier head interference (e.g., signal attenuation) with the induction system utilized to monitor wafer topography during the CMP process.
The limitations associated with conventional carrier head designs are not solely attributable to the bladder attachment means; e.g., known wafer ejectment systems have certain drawbacks as well. By ejecting a wafer in the manner described above, the bladder may place undue stress on inner portions of the wafer. Furthermore, expanding a central portion of the bladder to eject a supported wafer may create suction between the bladder and the wafer, which may ultimately prevent wafer ejection. As a still further limitation, conventional carrier head designs do not provide a large degree of bladder control proximate the outer peripheral edge of the bladder. Consequently, it is difficult to precisely control the planarization of the outer edge of the wafer (e.g., the outer 4-5 mm of a 300 mm wafer), which may result in lower die yields.
In view of the above, it should be appreciated that it would be desirable to provide a CMP carrier head suitable for planarizing a workpiece (e.g., a semiconductor wafer) that overcomes the limitations associated with conventional carrier head designs. In particular, it would be desirable if such a carrier head employed an improved bladder attachment design that utilizes less components, that facilitates refurbishing, and that permits components of the carrier head (e.g., the carrier housing) to be made of materials having lower compressive strengths (e.g., a polymer, such as plastic). In addition, it should be appreciated that it would be advantageous if such a carrier utilized an improved ejection system that did not unduly stress the wafer or create suction between the wafer and the bladder during ejectment. Finally, it should be appreciated that it would be desirable if such a carrier head included a system for providing improved bladder control proximate the outer edge of the wafer during planarization/polishing. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.